Sequential trait evolution did not drive deep-time diversification in sharks

Estimating how traits evolved and impacted diversification across the tree of life represents a critical topic in ecology and evolution. Although there has been considerable research in comparative biology, large parts of the tree of life remain underexplored. Sharks are an iconic clade of marine vertebrates, and key components of marine ecosystems since the early Mesozoic. However, few studies have addressed how traits evolved or whether they impacted their extant diversity patterns. Our study aimed to fill this gap by reconstructing the largest time-calibrated species-level phylogeny of sharks and compiling an exhaustive database for ecological (diet, habitat) and biological (reproduction, maximum body length) traits. Using state-of-the-art models of evolution and diversification, we outlined the major character shifts and modes of trait evolution across shark species. We found support for sequential models of trait evolution and estimated a small to medium-sized lecithotrophic and coastal-dwelling most recent common ancestor for extant sharks. However, our exhaustive hidden traits analyses do not support trait-dependent diversification for any examined traits, challenging previous works. This suggests that the role of traits in shaping sharks' diversification dynamics might have been previously overestimated and should motivate future macroevolutionary studies to investigate other drivers of diversification in this clade.

Drivers of the artiodactyl turnover in insular western Europe at the Eocene-Oligocene Transition

Simultaneously investigating the effects of abiotic and biotic factors on diversity dynamics is essential to understand the evolutionary history of clades. The Grande Coupure corresponds to a major faunal turnover at the Eocene-Oligocene transition (EOT) (~34.1 to 33.55 Mya) and is defined in western Europe as an extinction of insular European mammals coupled with the arrival of crown clades from Asia. Here, we focused on the species-rich group of endemic European artiodactyls to determine the drivers of the Grande Coupure during the major environmental disruptions at the EOT. Using Bayesian birth-death models, we analyzed an original high-resolution fossil dataset (90 species, >2,100 occurrences) from southwestern France (Quercy area) and estimated the regional diversification and diversity dynamics of endemic and immigrant artiodactyls. We show that the endemic artiodactyl radiation was mainly related to the Eocene tropical conditions, combined with biotic controls on speciation and clade-related diversity dependence. We further highlight that the major environmental changes at the transition (77% of species became extinct) and the concurrent increase in seasonality in Europe during the Oligocene were likely the main drivers of their decline. Surprisingly, our results do not support the widely-held hypothesis of active competition between endemic and immigrant artiodactyls but rather suggest a passive or opportunistic replacement by immigrants, which is further supported by morphological clustering of specific ecological traits across the Eocene-Oligocene transition. Our analyses provide insights into the evolutionary and ecological processes driving the diversification and decline of mammalian clades during a major biological and climatic crisis.

Shark mandible evolution reveals patterns of trophic and habitat-mediated diversification

Environmental controls of species diversity represent a central research focus in evolutionary biology. In the marine realm, sharks are widely distributed, occupying mainly higher trophic levels and varied dietary preferences, mirrored by several morphological traits and behaviours. Recent comparative phylogenetic studies revealed that sharks present a fairly uneven diversification across habitats, from reefs to deep-water. We show preliminary evidence that morphological diversification (disparity) in the feeding system (mandibles) follows these patterns, and we tested hypotheses linking these patterns to morphological specialisation. We conducted a 3D geometric morphometric analysis and phylogenetic comparative methods on 145 specimens representing 90 extant shark species using computed tomography models. We explored how rates of morphological evolution in the jaw correlate with habitat, size, diet, trophic level, and taxonomic order. Our findings show a relationship between disparity and environment, with higher rates of morphological evolution in reef and deep-water habitats. Deep-water species display highly divergent morphologies compared to other sharks. Strikingly, evolutionary rates of jaw disparity are associated with diversification in deep water, but not in reefs. The environmental heterogeneity of the offshore water column exposes the importance of this parameter as a driver of diversification at least in the early part of clade history.

Global impact and selectivity of the Cretaceous-Paleogene mass extinction among sharks, skates, and rays

The Cretaceous-Paleogene event was the last mass extinction event, yet its impact and long-term effects on species-level marine vertebrate diversity remain largely uncharacterized. We quantified elasmobranch (sharks, skates, and rays) speciation, extinction, and ecological change resulting from the end-Cretaceous event using >3200 fossil occurrences and 675 species spanning the Late Cretaceous-Paleocene interval at global scale. Elasmobranchs declined by >62% at the Cretaceous-Paleogene boundary and did not fully recover in the Paleocene. The end-Cretaceous event triggered a heterogeneous pattern of extinction, with rays and durophagous species reaching the highest levels of extinction (>72%) and sharks and nondurophagous species being less affected. Taxa with large geographic ranges and/or those restricted to high-latitude settings show higher survival. The Cretaceous-Paleogene event drastically altered the evolutionary history of marine ecosystems.

Combining palaeontological and neontological data shows a delayed diversification burst of carcharhiniform sharks likely mediated by environmental change

Estimating deep-time species-level diversification processes remains challenging. Both the fossil record and molecular phylogenies allow the estimation of speciation and extinction rates, but each type of data may still provide an incomplete picture of diversification dynamics. Here, we combine species-level palaeontological (fossil occurrences) and neontological (molecular phylogenies) data to estimate deep-time diversity dynamics through process-based birth-death models for Carcharhiniformes, the most speciose shark order today. Despite their abundant fossil record dating back to the Middle Jurassic, only a small fraction of extant carcharhiniform species is recorded as fossils, which impedes relying only on the fossil record to study their recent diversification. Combining fossil and phylogenetic data, we recover a complex evolutionary history for carcharhiniforms, exemplified by several variations in diversification rates with an early low diversity period followed by a Cenozoic radiation. We further reveal a burst of diversification in the last 30 million years, which is partially recorded with fossil data only. We also find that reef expansion and temperature change can explain variations in speciation and extinction through time. These results pinpoint the primordial importance of these environmental variables in the evolution of marine clades. Our study also highlights the benefit of combining the fossil record with phylogenetic data to address macroevolutionary questions.

Comment on "An early Miocene extinction in pelagic sharks"

[Figure: see text].

Fossilized cell structures identify an ancient origin for the teleost whole-genome duplication

Teleost fishes comprise one-half of all vertebrate species and possess a duplicated genome. This whole-genome duplication (WGD) occurred on the teleost stem lineage in an ancient common ancestor of all living teleosts and is hypothesized as a trigger of their exceptional evolutionary radiation. Genomic and phylogenetic data indicate that WGD occurred in the Mesozoic after the divergence of teleosts from their closest living relatives but before the origin of the extant teleost groups. However, these approaches cannot pinpoint WGD among the many extinct groups that populate this 50- to 100-million-y lineage, preventing tests of the evolutionary effects of WGD. We infer patterns of genome size evolution in fossil stem-group teleosts using high-resolution synchrotron X-ray tomography to measure the bone cell volumes, which correlate with genome size in living species. Our findings indicate that WGD occurred very early on the teleost stem lineage and that all extinct stem-group teleosts known so far possessed duplicated genomes. WGD therefore predates both the origin of proposed key innovations of the teleost skeleton and the onset of substantial morphological diversification in the clade. Moreover, the early occurrence of WGD allowed considerable time for postduplication reorganization prior to the origin of the teleost crown group. This suggests at most an indirect link between WGD and evolutionary success, with broad implications for the relationship between genomic architecture and large-scale evolutionary patterns in the vertebrate Tree of Life.

Durnonovariaodus maiseyi gen. et sp. nov., a new hybodontiform shark-like chondrichthyan from the Upper Jurassic Kimmeridge Clay Formation of England

A partial skeleton of a hybodontiform shark-like chondrichthyan from the Upper Jurassic Kimmeridge Clay Formation of Dorset, England, is described and designated as a new genus and species, Durnonovariaodus maiseyi gen. et sp. nov. The holotype and only known specimen, which is represented by disarticulated splanchnocranial elements with associated teeth, a single dorsal fin spine, the pelvic girdle, as well as unidentifiable cartilage fragments, plus countless dermal denticles, exhibits a puzzling combination of dental and skeletal features, providing important new insights into the morphological and ecological diversity of hybodontiforms. Durnonovariaodus gen. nov. displays a unique set of dental characters, showing close morphological resemblance to Secarodus from the Middle Jurassic of England, which was erected for distinctive, strongly labio-lingually compressed multicuspid cutting teeth originally described as Hybodus polyprion. Skeletally, Durnonovariaodus gen. nov. resembles Hybodus and Egertonodus in having a palatoquadrate with a palatobasal process and an ethmoidal articular surface, combined with the possession of dorsal fin spines ornamented with costae. Therefore, and given the absence of any conclusive phylogenetic framework, Durnonovariaodus maiseyi gen. et sp. nov. is here tentatively referred to Hybodontidae until more complete material becomes available in order to enable a more reliable suprageneric identification. The holotype of Durnonovariaodus maiseyi gen. et sp. nov. contains two separate pelvic half-girdles, a feature previously considered as evolutionarily primitive among hybodontiforms. However, unfused pelvic half-girdles also occur in the supposedly closely related species Hybodus hauffianus and may in fact have been more widely distributed among hybodontiforms than previously thought, thus rendering the phylogenetic utility of separated pelvic half-girdles for inferring hybodontiform interrelationships difficult and unresolved.